AskDefine | Define antigen

Dictionary Definition

antigen n : any substance (as a toxin or enzyme) that stimulates the production of antibodies

User Contributed Dictionary



Contraction of antibody-generator.


  1. A substance that induces an immune response, usually foreign.


substance that induces an immune response
  • Finnish: antigeeni
  • Romanian: antigen
  • Swedish: antigen

Related terms

Extensive Definition

An antigen (from antibody-generating) or immunogen is a substance that prompts the generation of antibodies and can cause an immune response. The word originated from the notion that they can stimulate antibody generation. We now know that the immune system does not consist of only antibodies. The modern definition encompasses all substances that can be recognized by the adaptive immune system.
Antigens are usually proteins or polysaccharides. This includes parts (coats, capsules, cell walls, flagella, fimbrae, and toxins) of bacteria, viruses, and other microorganisms. Lipids and nucleic acids are antigenic only when combined with proteins and polysaccharides. Non-microbial exogenous (non-self) antigens can include pollen, egg white, and proteins from transplanted tissues and organs or on the surface of transfused blood cells.
  • Tolerogen - A substance that invokes a specific immune non-responsiveness due to its molecular form. If its molecular form is changed, a tolerogen can become an immunogen.
  • Allergen - An allergen is a substance that causes the allergic reaction. The (detrimental) reaction may result after exposure via ingestion, inhalation, injection, or contact with skin.
Cells present their antigens to the immune system via a histocompatibility molecule. Depending on the antigen presented and the type of the histocompatibility molecule, several types of immune cells can become activated.

Origin of antigens

Antigens can be classified in order of their origins.

Exogenous antigens

Exogenous antigens are antigens that have entered the body from the outside, for example by inhalation, ingestion, or injection. By endocytosis or phagocytosis, these antigens are taken into the antigen-presenting cells (APCs) and processed into fragments. APCs then present the fragments to T helper cells (CD4+) by the use of class II histocompatibility molecules on their surface. Some T cells are specific for the peptide:MHC complex. They become activated and start to secrete cytokines. Cytokines are substances that can activate cytotoxic T lymphocytes (CTL), antibody-secreting B cells, macrophages, and other particles.

Endogenous antigens

Endogenous antigens are antigens that have been generated within the cell, as a result of normal cell metabolism, or because of viral or intracellular bacterial infection. The fragments are then presented on the cell surface in the complex with MHC class I molecules. If activated cytotoxic CD8+ T cells recognize them, the T cells begin to secrete various toxins that cause the lysis or apoptosis of the infected cell. In order to keep the cytotoxic cells from killing cells just for presenting self-proteins, self-reactive T cells are deleted from the repertoire as a result of tolerance (also known as negative selection).


An autoantigen is usually a normal protein or complex of proteins (and sometimes DNA or RNA) that is recognized by the immune system of patients suffering from a specific autoimmune disease. These antigens should, under normal conditions, not be the target of the immune system, but, due to mainly genetic and environmental factors, the normal immunological tolerance for such an antigen has been lost in these patients.

Tumor antigens

Tumor antigens or Neoantigens are those antigens that are presented by MHC I or MHC II molecules on the surface of tumor cells. These antigens can sometimes be presented by tumor cells and never by the normal ones. In this case, they are called tumor-specific antigens (TSAs) and, in general, result from a tumor-specific mutation. More common are antigens that are presented by tumor cells and normal cells, and they are called tumor-associated antigens (TAAs). Cytotoxic T lymphocytes that recognize these antigens may be able to destroy the tumor cells before they proliferate or metastasize.
Tumor antigens can also be on the surface of the tumor in the form of, for example, a mutated receptor, in which case they will be recognized by B cells.


A native antigen is an antigen which isn't yet processed by an APC to smaller parts. T cells cannot bind native antigens, but require that they are processed by APCs, while B cells can be activated by native ones.
antigen in Afrikaans: Antigene
antigen in Bulgarian: Антиген
antigen in Catalan: Antigen
antigen in Czech: Antigen
antigen in Danish: Antigen
antigen in German: Antigen
antigen in Spanish: Antígeno
antigen in Basque: Antigeno
antigen in French: Antigène
antigen in Korean: 항원
antigen in Indonesian: Antigen
antigen in Italian: Antigene
antigen in Hebrew: אנטיגן
antigen in Lithuanian: Antigenas
antigen in Dutch: Antigeen
antigen in Japanese: 抗原
antigen in Norwegian: Antigen
antigen in Polish: Antygen
antigen in Portuguese: Antígeno
antigen in Romanian: Antigen
antigen in Russian: Антиген
antigen in Simple English: Antigen
antigen in Slovak: Antigén
antigen in Slovenian: Antigen
antigen in Serbian: Антиген
antigen in Serbo-Croatian: Antigen
antigen in Finnish: Antigeeni
antigen in Swedish: Antigen
antigen in Vietnamese: Kháng nguyên
antigen in Ukrainian: Антиген
antigen in Chinese: 抗原

Synonyms, Antonyms and Related Words

Rh factor, Rh-negative, Rh-positive, Rh-type, Rhesus factor, acquired immunity, active immunity, agglutinin, allergen, anaphylactin, antiantibody, antibody, antiserum, antitoxic serum, antitoxin, antivenin, arterial blood, artificial immunity, blood, blood bank, blood cell, blood count, blood donor, blood donor center, blood group, blood grouping, blood picture, blood platelet, blood pressure, blood serum, blood substitute, bloodmobile, bloodstream, circulation, clinical dextran, congenital immunity, dextran, erythrocyte, familial immunity, globulin, gore, grume, hematics, hematologist, hematology, hematoscope, hematoscopy, hemocyte, hemoglobin, hemometer, humor, ichor, immunity, immunization, incomplete antibody, inherent immunity, inherited immunity, interferon, isoantibody, leukocyte, lifeblood, natural immunity, neutrophil, nonspecific immunity, nonsusceptibility to disease, opsonic immunity, opsonin, passive immunity, phagocyte, phagocytic immunity, plasma, plasma substitute, precipitin, racial immunity, red corpuscle, resistance, serum, specific immunity, toxin-antitoxin immunity, type O, venous blood, white corpuscle
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